MongoMapper
http://wiki.github.com/jnunemaker/mongomapper/
MongoMapper is a Ruby wrapper library which aims to make using MongoDB much easier and friendlier than the default Ruby driver provided by XGen. When it makes sense to do so, MongoMapper tries to stick closely with the familiar syntax of ActiveRecord.
From the Wiki:
Ruby ORMs
Thanks!
Data Driven Applications 
with Ruby and MongoDB
http://try.mongodb.org/
Mongoid
http://mongoid.org
Mongoid provides an elegant way to persist and query Ruby objects to documents in MongoDB. 
FEATURES:

- Effortless mapping of Ruby objects to documents and embedded documents.
- Chained criteria.
- Chained scopes.
- Support for versioning of documents.
From the Home Page:
Candy
http://github.com/SFEley/candy
"Mongo like candy!" -- Blazing Saddles

Candy's goal is to provide the simplest possible object persistence for the MongoDB database. By "simple" we mean "nearly invisible." Candy doesn't try to mirror ActiveRecord or DataMapper. Instead, we play to MongoDB's unusual strengths -- extremely fast writes and a set of field-specific update operators -- and do away with the cumbersome, unnecessary methods of last-generation workflows.

Methods like find.  Or save.
From the README:
Overview
Thanks Kyle Banker & 10Gen!
Why MongoDB?
Jim Mulholland
Squeejee
http://squeejee.com
John Taber
Tiger Nassau
http://tigernassau.com